The collection of solar energy by means of heliostats, which concentrate the radiation on reflector-mirrors, is known technique. The latter, in turn, convey the radiation on thermal storage and exchange devices based upon fluidized particle beds. A system of this type is described, for example, in WO2013/150347A1 in the name of the same owner.
Plants for the production of thermal/electrical energy can be based upon such devices for storage and exchange of thermal energy of solar origin, which plants will include one or more units for storage and/or exchange according to the thermal power one wants to obtain.
The fluidized-bed devices of known technique are implemented according to two main structures.
Based upon a first structure, described in WO2013/150347A1, the solar radiation is received on the walls of a metal cavity of the device. Such cavity defines a portion of the casing of the bed of particles and it extends inside the latter. The fluidized bed of particles subtracts the thermal energy deriving from the concentrated solar radiation from the cavity walls.
In presence of high incident radiative flows, the just described structure has the drawback of exposing the cavity surface to high thermal temperatures and gradients which could compromise the thermo-mechanical resistance and durability. In order to lower and control the thermal flows thereto the cavity walls are exposed, the heliostat field can be organized in several sub-sections arranged around the device and configured to uniform the thermal flows on the cavity surface. However, such configuration of the heliostat field requests a considerable ground occupation for each solar generation unit.
Furthermore, the described structure puts limits to the maximum operating temperature of the storage and exchange device, as this depends upon the thermal resistance of the material constituting the cavity walls. Such operating temperature is also conditioned by the mode for transferring the thermal energy from the cavity to the bed of particles and by the conductivity of the material constituting the cavity itself.
In a second known structure, the above-mentioned cavity is not provided and the bed of particles of the storage and exchange device receives the solar radiation concentrated through a window of transparent material, typically quartz, obtained on the casing of the device.
However, a criticality of such second structure consists in that the direct contact of the transparent window with the fluidized solid has to be avoided, and this to limit the appearance, in time, of delustring phenomena of the transparent material which reduce the reception effectiveness thereof.
An additional disadvantage related to the use of receiving means of the type with transparent window is related to the difficulty of producing windows in quartz with larger sizes than those used for plants of laboratory or of prototype kind.
Moreover, an additional drawback associated to both above-mentioned structures—and in particular to related receiving means with cavities or windows—consists in the thermal losses due to the re-release towards the outer environment of a portion of the incident solar energy. Such portion depends upon the features of the material constituting the receiving means.
As a consequence of what just noted, the above-mentioned devices for storing and transferring thermal energy of solar origin can have high costs for producing the electrical energy, however far from a so-called “parity grid”.